Architecture of an acoustic multistatic system

ABSTRACT

The present invention relates to a multistatic sonar system having a particular mode of communication between the emitter base and the receiver bases responsible for detecting the sonar echoes. This system makes it possible in particular to circumvent the drawbacks occasioned by the use of radio communications or of satellite-based communications generally used. The signal emitted by the emitter base comprises an apprisal pulse conveying information pertaining to the mode of operation of the system according to the invention, in particular the instant of emission, and a sonar pulse proper. This system is especially intended for use by underwater vessels while diving, for detecting and locating surrounding objects or controlling the positioning of an autonomous underwater vehicle.

BACKGROUND OF THE INVENTION

The present invention relates to a multistatic sonar system in which thecommunication between the emitter base and the receiver bases,responsible for detecting the sonar echoes, is done acoustically. Thissystem makes it possible in particular to circumvent the drawbacksoccasioned by the use of radio communications or of satellite-basedcommunications, generally used to synchronize the emission and receptionfunctions. This system is intended in particular for use by underwatervessels while diving.

An important characteristic of multistatic sonar systems consists in thephysical separation that exists between the emitter base from which thesonar pulse originates and the receiver bases responsible for detectingand locating sonar echoes. Such systems have the advantage in particularof possessing a flexible and modular architecture, the complexity ofwhich may be tailored to the geographical and operational situation. Themultistatic sonar thus formed can comprise, as the case may be, avariable number of receiver bases. On the other hand such anarchitecture requires the existence of means of communication betweenthe emitter base which synthesizes and emits the sonar pulse and thereceiver bases which receive the echoes. Specifically, in so far as onewishes to achieve coherent detection, the receiver bases must beinformed of the operating parameters of the system. These parametersprovide details regarding for example the type of pulse emitted, thefrequency of emission, the period of repetition or else regarding theinstant of emission. Other information such as the position of theemitter base and that of the receiver bases, which makes it possible toreference the measurements of the instants of arrival of the echoes,also passes via these means of communication.

To establish a communication between the emitter base and the receiverbases, it is well-known current practice to use a radio link, forexample an RF or satellite based link.

The information transmitted is generally of two types:

context information relating to the nature of the pulse emitted (coding,duration, etc.), the position of the emitter base, the generalconfiguration of the sonar system or else date information,

synchronization information making it possible to accurately ascertainthe instant of emission of the sonar pulse and to calculate the distanceseparating the objects having returned echoes from the receiver bases.

Subsequently in the document this information will be referred to asmode information.

The use of a radio link has the advantage of allowing the simultaneoustransmission of a large amount of information, the bandwidth of thesignal emitted possibly being considerable. On the other hand, in acertain number of circumstances, a radio link turns out to be difficultor even impossible to use. Such is the case in particular in respect ofunderwater craft when they are diving. Such is also the case for examplewhen the operational context excludes the use of radio emissions, forreasons of stealth.

SUMMARY OF THE INVENTION

The subject of the invention alleviates these drawbacks by proposing amultistatic sonar system in which the transmission of the operatinginformation between the emitter base and the receiver bases is performedby means of an acoustic signal.

The system according to the invention has, among other advantages, thatof operating regardless of the depth of submersion of the emitter baseand receiver bases and may therefore be implemented by submarines whilediving. The present invention also has the advantage of allowing thereal-time transmission of operating information. This information istransmitted in tempo with the sonar pulses. The acoustic signal carryingthe operating information and the sonar pulse, are advantageouslytransmitted by a single emitter device, the two signals being separatedby a time interval Δt which may be variable and whose value may becontained in the operating information. The latter information maymoreover be encrypted.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages will become apparent in the wake ofthe description illustrated by FIGS. 1 to 3 which represent:

FIG. 1, an illustration of a multistatic sonar system according to theprior art,

FIG. 2, an illustration of a system according to the invention,

FIG. 3, an illustration of the structure of the acoustic signal emittedby the system according to the invention.

DETAILED DESCRIPTION

The illustrations of FIGS. 1 to 3 are presented by way of example so asto facilitate the description of the invention. They do not of courserepresent a limitation as to the structure or to the embodiment of thesystem according to the invention.

FIG. 1 illustrates a typical multistatic sonar system structure knownfrom the prior art.

This structure comprises an emitter base and one or more receiver bases.For reasons of clarity FIG. 1 depicts just one receiver base.

The emitter base 11 chiefly comprises two subassemblies, a subassemblyintended for emitting the sonar pulse and a subassembly intended fortransmitting the operating information. These two subassemblies aremoreover managed by the sonar management facility 12.

The subassembly intended for emitting the sonar pulse comprises in aconventional manner a sonar emitter 13 and an acoustic antenna 14.

The subassembly intended for transmitting the mode information comprisesa radio emitter 15 and an antenna 16.

In symmetric fashion, each receiver base 17 comprises a subassembly forreceiving the sonar echoes 113 reflected by the objects 114 lying in thezone covered by the sonar emission 112. This subassembly itselfcomprises an acoustic antenna 18 and a passive sonar receiver 19.

The receiver base 17 also comprises a subassembly for receiving theoperating information itself comprising an antenna 110 and a radioreceiver 111.

The illustration of FIG. 1 makes it possible to appreciate thelimitations of such a structure. In particular, the use of an electricalradio link 115 for the communications between the emitter base and thereceiver bases necessarily implies that at least the elements 16 and 20remain on the surface. The system is therefore hardly compatible withbases 11 and 17 carried by underwater craft deploying at depth.Moreover, it may be noted that even under operational conditions, in sofar as the emitter base must be capable of communicating with severalreceiver bases, it is difficult to use a highly directional radioemission. The communication of the emitter base with the receiver basestherefore brings about an emission that is sensitive to meteorologicalconditions, that is not very stealthy and can be detected by hostileelements. This results in significant risks of jamming or ofdisturbance. This absence of stealth which reveals the presence of thesystem may render this system ineffective.

Additionally a structure such as illustrated by FIG. 1 turns out to becomplex in its structure, the transmission of the operating informationrequiring a transmission chain 15, 16 specific to this function andindependent of the emission of the sonar pulse.

FIG. 2 diagrammatically presents the base structure of the systemaccording to the invention. Like that described in FIG. 1, this systemcomprises an emitter base 21 and generally several receiver bases 22. Onthe other hand, in contradistinction to the system illustrated by FIG.1, the base 21 communicates with the bases 22 via an acoustic link. Themode information and the sonar pulse are thus transmitted via the samechannel.

For this purpose the emitter base 21 comprises a modulator 23, making itpossible to generate a modulated acoustic pulse 210. This pulse ismodulated by the mode information coded by means of the coding device24. The coding of the information may for example consist in theproduction of a binary message comprising the number of bits requiredfor the representation of the information. The modulator 23 makes itpossible to modulate the acoustic pulse by means of the signalconsisting of the binary message. The modulation produced may forexample consist of a modulation by frequency hopping or phase hopping.The modulated acoustic pulse, carrying the mode information, is alsoreferred to as the apprisal pulse.

The emitter base 21 also comprises a summator device 225 which makes itpossible to calculate the sum of the apprisal pulse and of the sonarpulse 211 provided by the management device 12. It finally comprises asonar emitter 13 and an acoustic antenna 14 which emit the sum signal212 into the propagation medium.

In an analogous manner, the structure of each receiver base is tailoredto the reception of mode information and of the sonar echoes on one andthe same reception channel. For this purpose the base comprises a singleacoustic antenna 18 which transforms the signals received intoelectrical signals 24. The signals received correspond on the one handto the information messages which come from the emitter base by directtransmission 215 and on the other hand to the echoes 217 reflected bythe objects 218 that may lie in the zone covered by the sonar emission216. The receiver base also comprises a separator device 26 making itpossible to separate the sonar echoes from the apprisal pulses. It alsocomprises a demodulator 27 and a decoding device 28 making it possibleto recover the mode information required to configure the receiver 19 insuch a way as to optimize the reception of the sonar echoes. It finallycomprises processing means 29 which process the sonar echoes received insuch a way as to determine the position of the objects 114.

According to a variant structure, in the case of an acoustic antennacomprising several hydrophones, it is possible to assign morespecifically certain hydrophones to the reception of the apprisal pulse.The antenna of the receiver base is then split into two subassemblies.This structure may culminate in the complete separation of the pathwaysfor receiving the apprisal pulses and for receiving the sonar echoes,the receiver base then no longer comprises any separator device 26.

As may be noted in FIG. 2, the structure of the system according to theinvention has the advantage of not comprising any electrical radio linkand hence of being able to operate in total submersion. It also makes itpossible to pass the mode information and the sonar pulse through oneand the same channel and to thus ensure temporal concordance between thetwo signals.

The structure of the system according to the invention mayadvantageously be supplemented with encryption/decryption meansassociated with the means of coding 24 and of decoding 28 of the modeinformation. These means have the effect of preventing direct reading ofthe mode information by third parties.

FIG. 3 diagrammatically presents in a general manner the structure ofthe signal utilized by the system according to the invention. The signalpresented in FIG. 3 is emitted by the emitter base 21 destined for thereceiver bases. It comprises in particular the apprisal pulse 31 whichcontains the mode information which follows the direct path 215 of FIG.2.

The signal emitted also comprises the sonar pulse 32 proper. In FIG. 3the two pulses which make up the emission signal are spaced apart by atime interval Δt. This time interval allows in particular the variousreceiver bases making up the system to receive and to decode theapprisal pulse before receiving an echo of the sonar pulse. The modeinformation carried by the apprisal pulse is utilized by the receiverbases to configure their receiver in accordance with the shape of thepulse emitted.

The duration Δt may be variable as a function of the applications. Itmay for example have been predetermined for each mode of operation andbe known to the receiver bases. It may indeed be fixed during theoperation of the system and be transmitted to the receiver bases as modeinformation. The constraint on the value of Δt is in particular imposedby the time required by the receiver bases to configure themselvesbefore receiving the echoes from the sonar pulse and to take intoaccount the durations of the required acoustic paths.

In a particular embodiment, a zero interval Δt may even be envisaged.The two pulses are then immediately consecutive.

As illustrated in FIG. 3, the signal emitted by the emitter base is, inthe case of the system according to the invention, a composite signalformed of two pulses of different structures.

The sonar pulse is a pulse of conventional shape, known elsewhere. Itmay for example be of fixed frequency or else assigned a particularmodulation law.

The apprisal pulse for its part forms the subject of a particularmodulation. The modulating signal represents the whole set of modeinformation to be transmitted. It consists for example of theconcatenation of the data pertaining to the mode information, put intobinary form. These concatenated data form a word or a frame comprising agiven number of bits dependent on the nature of the informationtransmitted.

This signal will modulate the apprisal pulse through a digitalmodulation device, of modem type for example. As stated previously, themodulation code may for example be a coding by phase or frequencyhopping, phase and frequency hopping or any other digital coding.

The duration of the apprisal pulse is dependent on the amount of modeinformation to be transmitted to the receiver bases. In practice theduration of the apprisal pulse is of the order of a few seconds. Thisduration which corresponds to the time required to transmit a fewhundred to a few thousand bits of information, is compatible with theconventional bandwidths of the sonar emitters. The system according tothe invention therefore advantageously does not require the use ofparticular means of emission. Thus, for example, an emitter having abandwidth equal to 1 kHz will be able to transit an apprisal pulse of 1s conveying a mode message of a length equal to around 1 kbits.

The manner of operation of the system according to the invention may bedescribed with the aid of FIGS. 2 and 3. The signal emitted by theemitter base is a signal comprising two pulses: an apprisal pulse 31 anda sonar pulse 32. The emitter base may for example be the active sonarof a submarine. The apprisal pulse is received in direct fashion by thereceiver bases. Each of the receiver bases is moreover equipped in aknown manner, with means allowing it to ascertain its position withrespect to the emitter base. Each base carries out the decoding of themode information. This information comprises in particular the value Δtof the time which separates the instant of emission of the apprisalpulse, from that of emission of the sonar pulse. Knowledge of Δt allowsthe receiver bases to ascertain the instant of emission of the sonarpulse and to perform a measurement of the delay between the pulseemitted and the instant of arrival of an echo. The assembly of receiverbases thus behaves like a multistatic active sonar.

In the illustration of FIG. 2, the emitter base is portrayed ascomprising a single sonar emitter, the apprisal pulse and the sonarpulse being emitted by one and the same emitter 13. This solutionrepresents an economical embodiment in terms of bulkiness and cost.Nevertheless this solution is not limiting and it is of course possibleto use two different acoustic emitters if the need to have two separateemitters is apparent.

According to an embodiment of the system, the instant of emission of thesonar pulse is deduced from the instant of reception of the apprisalpulse. The receiver bases perform for example a detection of theenvelope of the apprisal pulse and allocate the sonar pulse an instantof emission dependent on the instant of reception of the apprisal pulseand Δt.

According to another embodiment, the receiver bases and the emitter baseare previously synchronized. The mode data thus integrate informationproviding the precise date of emission of the sonar pulse, therebyallowing the receiver bases to ascertain the instant of emission of thesonar pulse.

The structure of the signal emitted has the advantage of rendering thesystem parametrizable and hence easily tailored. In particular, byvirtue of the information that can be transmitted through the apprisalpulse, it makes it possible to modify the configuration of the system inreal time. As a function of the information transmitted, it is forexample possible to alter the frequency of emission of the sonar pulse,its type of modulation or else the value of the gap Δt which separatesthe apprisal pulse from the sonar pulse: protection against such a sonaris thus rendered difficult. It is moreover possible as illustrated byFIG. 2, to perform the encrypting of the mode information beforemodulation. In this way, in the absence of the decryption key, it isimpossible to gain access to the mode parameters of the system.

In the above description the mode of emission described comprises asignal emitted systematically composed of two pulses. Such a signal isrequired in particular in the case where one wishes to be able to modifycertain mode information from one emission to the next. It isnevertheless possible to envisage simpler modes of operation, for whichthe operating parameters remain unchanged over a relatively long time.In this case it is for example possible to design a system for which theemission signal is sometimes composed of an apprisal pulse followed byan emission pulse, and sometimes simply of an emission pulse. It is alsopossible to design a system for which the emission signal is composed ofan apprisal pulse followed by several emission pulses. The patternemitted by the system according to the invention can take various formsas a function of the applications.

As was stated previously the system according to the invention appliesin particular to the embodying of a multistatic sonar intended forlocating echoes coming from objects situated in a zone undersurveillance. However, this application is not the only one that may beenvisaged and in no way limits the field of use of the system. It is forexample also possible to use it to carry out the positioning ofautonomous underwater vehicles. In this type of application, the emitterbase is situated on the autonomous craft and the receiver bases are forexample positioned on surface vessels.

1-12. (canceled)
 13. A multistatic acoustic system, comprising: anemitter base comprising means of emission of sonar pulses; a receiverbase comprising means of reception and of processing of sonar echoes;the emitter base also comprising means for transmitting mode informationto the receiver base wherein the mode information is transmitted to thereceiver base in acoustic form by an appraisal pulse, said appraisalpulse being modulated by a signal containing said mode information. 14.The system as claimed in claim 13, wherein an appraisal pulse and asonar pulse are emitted by a single emitter successively emitting thetwo pulses.
 15. The system as claimed in claim 13, wherein a sonar pulseand an appraisal pulse are separated by a time interval Δt dependent onthe mode of operation of the system and known to the receiver bases. 16.The system as claimed in claim 13, wherein, Δt being equal to zero, twopulses are emitted so as to be immediately consecutive.
 17. The systemas claimed in claim 14, wherein, Δt being equal to zero, the two pulsesare emitted so as to be immediately consecutive.
 18. The system asclaimed in claim 13, wherein a sonar pulse and an appraisal pulse areseparated by a time interval Δt dependent on the mode of operation ofthe system and known to the receiver bases.
 18. The system as claimed inclaim 14, wherein a sonar pulse and an appraisal pulse are separated bya time interval Δt dependent on the mode of operation of the system andknown to the receiver bases.
 19. The system as claimed in claim 13,wherein a sonar pulse and an appraisal pulse are separated by a timeinterval Δt whose duration is transmitted to the receiver bases with themode information.
 20. The system as claimed in claim 14, wherein a sonarpulse and an appraisal pulse are separated by a time interval Δt whoseduration is transmitted to the receiver bases with the mode information.21. The system as claimed in claim 13, wherein the modulation of theappraisal pulse by the modulating signal containing the mode informationis carried out by digital coding.
 22. The system as claimed in claim 14,wherein the modulation of the appraisal pulse by the modulating signalcontaining the mode information is carried out by digital coding. 23.The system as claimed in claim 21, wherein the modulation of theappraisal pulse by the modulating signal containing the mode informationis carried out by phase-hopping coding.
 24. The system as claimed inclaim 22, wherein the modulation of the appraisal pulse by themodulating signal containing the mode information is carried out byphase-hopping coding.
 25. The system as claimed in claim 21, wherein themodulation of the appraisal pulse by the modulating signal containingthe mode information is carried out by frequency-hopping coding.
 26. Thesystem as claimed in claim 22, wherein the modulation of the appraisalpulse by the modulating signal containing the mode information iscarried out by frequency-hopping coding.
 27. The system as claimed inclaim 13, wherein the instant of emission of the sonar pulse isdetermined on the basis of the instant of reception of the appraisalpulse by the receiver bases and of the mode of operation.
 28. The systemas claimed in claim 14, wherein the instant of emission of the sonarpulse is determined on the basis of the instant of reception of theappraisal pulse by the receiver bases and of the mode of operation. 29.The system as claimed in claim 13, wherein the instant of emission ofthe sonar pulse is determined on the basis of the date of emission ofsaid sonar pulse, contained in the mode message.
 30. The system asclaimed in claim 14, wherein the instant of emission of the sonar pulseis determined on the basis of the date of emission of said sonar pulse,contained in the mode message.
 31. The system as claimed in claim 13,further comprising means for encrypting the mode information.
 32. Aprocess for controlling the position of an autonomous underwater craft,said process using the system as claimed in claim
 17. 33. A process forcontrolling the position of an autonomous underwater craft, said processusing the system as claimed in claim 18.